Recent years have seen a strong resurgence of interest in biological electron microscopy (EM)including cryo-electron tomography. A limitation of EM analysis, in particular in cellular samples, is determing the location of a protein of interest. Our ultimate goal is to develop methods that will combine the reliable preservation of cell structure based on rapid freezing and vitrification with labeling technologies that give sufficient signal-to-noise so that these labels are readily visible by EM, particularly in 3D electron tomograms. We propose to develop a metallothionein gene as a "clonable tag" that will bind gold to enhance its density in a variety of samples for EM. Metallothioneins are small proteins (~6.5 kD) that are avid metal binders and that have been shown to form gold clusters in vitro (Mercogliano &DeRosier 2006. J Mol Biol. 355:211-23). Such a clonable high-density tag would revolutionize the utility of cellular tomography because the 3D position of proteins in complex cellular structures could be determined by tomography at nanometer resolution. The utility of metallothionein as a clonable tag will be explored in two aims The first aim is to develop metallothionein as a clonable label for cryo-electron microscopy and cryo-electron tomography applied to isolated or in vitro reconstituted macromolecular assemblies. In particular, the microtubule-Eg5 motor complex will be used for qualitative and quantitative assessment of the metallothionein labeling properties. The usefulness of metallothionein as a directly visible density marker in averagable and non-averagable structures will be assayed. Metallothionein-tagged cellular components in vitrified sections of intact cells, liklely with the use of silver-enhancement will also be tested. The second aim is to develop metallothionein as a clonable density tag for protein localization in rapidly frozen and freeze-substituted material embedded in plastic. The metallothionein-tagged Eg5 kinesin will be localized spindles assembled in vitro using Xenopus egg extracts, as well as in vertebrate tissue culture cells which are suitable for tomography. Finally, the metallothionein tag will be used in budding yeast on a variety of proteins, including alpha-tubulin in rnicrotubules, the Cin8 kinesin-like motor protein, and Spc42, a very abundant spindle pole component. It is anticipated that the metallothionein clonable density tag will be useful for a variety of EM techniques.